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Discusses oral glucose tolerance test that measures glucose (blood sugar) levels. Explains that test checks for gestational diabetes, prediabetes, and diabetes. Covers the types of tests done and how to prepare for them.

Discusses oral glucose tolerance test that measures glucose (blood sugar) levels. Explains that test checks for gestational diabetes, prediabetes, and diabetes. Covers the types of tests done and how to prepare for them.

Oral Glucose Tolerance Test

Test Overview

The oral
glucose tolerance test (OGTT) measures the body's ability to use a type of
sugar, called glucose, that is the body's main source of energy. An OGTT can be
used to diagnose
prediabetes and
diabetes. An OGTT is most commonly done to check for
diabetes that occurs with pregnancy (gestational diabetes).

Why It Is Done

The oral glucose tolerance test (OGTT)
is done to:

Check pregnant women for gestational diabetes.
You
have an increased chance of developing gestational diabetes if you:

Have had gestational diabetes during a
previous pregnancy.

Have previously given birth to a baby who
weighed more than 9 lb (4.1 kg).

Are younger than age 25 and were
overweight before getting pregnant.

Diagnose prediabetes and diabetes.

How To Prepare

Glucose tolerance diagnostic test

To prepare for
the glucose tolerance diagnostic test:

Eat a balanced diet that contains at least
150 grams (g) of carbohydrate per day for 3 days before the test.
Fruits, breads, cereals, grains, rice, crackers, and starchy vegetables such as
potatoes, beans, and corn are good sources of carbohydrate.

Do not
eat, drink, smoke, or exercise strenuously for at least 8 hours before your
first blood sample is taken.

Tell your doctor about
all prescription and nonprescription medicines you are taking. You may be
instructed to stop taking certain medicines before the test.

The glucose tolerance diagnostic test may take up to 4
hours. Since activity can interfere with test results, you will be asked to sit
quietly during the entire test. Do not eat during the test. You may drink only
water during this time.

Talk to your doctor about any
concerns you have regarding the need for the test, its risks, or how it will be
done. To help you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).

How It Is Done

Glucose tolerance diagnostic test

On the day of
testing, the following steps will be done:

A blood sample will be collected when you
arrive. This is your fasting blood glucose value. It provides a baseline for
comparing other glucose values.

You will be asked to drink a sweet
liquid containing a measured amount of glucose. It is best to drink the liquid
quickly. For the standard glucose tolerance test, you will drink 75
grams or 100 grams.

Blood samples will be collected at
timed intervals of 1, 2, and sometimes 3 hours after you drink the glucose. Blood samples
may also be taken as soon as 30 minutes to more than 3 hours after you drink
the glucose.

Blood test

The health professional taking a sample
of your blood will:

Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.

Clean the needle site with
alcohol.

Put the needle into the vein. More than one needle stick
may be needed.

Attach a tube to the needle to fill it with
blood.

Remove the band from your arm when enough blood is
collected.

Apply a gauze pad or cotton ball over the needle site as
the needle is removed.

Apply pressure to the site and then a
bandage.

How It Feels

You may find it hard to drink the
extremely sweet glucose liquid. Some people feel sick after drinking the
glucose liquid and may vomit. Vomiting may prevent you from completing the test
on that day.

The blood sample is taken from a vein in your arm. An
elastic band is wrapped around your upper arm. It may feel tight. You may feel
nothing at all from the needle, or you may feel a quick sting or pinch.

You may feel faint from having several blood samples taken in one day.
But the amount of blood taken will not cause significant blood loss or
anemia.

Risks

Some people's blood glucose levels drop very low
toward the end of the test. But some people feel like their sugar levels
are low, when their levels actually are not low. Symptoms of low blood glucose
include weakness, hunger, sweating, and feeling nervous or restless. If you
develop these symptoms during the test, you may have your sugar level checked
quickly with a glucose meter. If your level is very low, the test will be
stopped.

There is very little risk of a problem from having blood
drawn from a vein.

You may develop a small bruise at the puncture
site. You can reduce the risk of bruising by keeping pressure on the site for
several minutes after the needle is withdrawn.

In rare cases, the
vein may become inflamed after the blood sample is taken. This condition is
called phlebitis and is usually treated with a warm compress applied several
times daily.

Continued bleeding can be a problem for people with
bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning
medicines can also make bleeding more likely. If you have bleeding or clotting
problems, or if you take blood-thinning medicine, tell your health professional
before your blood is drawn.

Results

The
oral glucose tolerance test (OGTT) measures the body's
ability to use a type of sugar, called glucose, that is the body's main source
of energy.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Normal glucose tolerance test values

75 g of
glucose

Fasting:

Less than or equal to 100 milligrams per deciliter (mg/dL) or 5.6 millimoles per liter (mmol/L)1

Many conditions can change blood glucose levels. Your
doctor will discuss any significant abnormal results with you in
relation to your symptoms and past health.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

Smoking.

Alcohol.

Recent surgery, illnesses, and infectious diseases.

Weight loss through dieting.

Long periods of bed rest (such as from a hospitalization or illness).

What To Think About

When screening for gestational diabetes, a test might be done before the 100-gram oral glucose tolerance test (OGTT). For the first test, you will not need to fast. You will drink 50 grams of glucose. Your blood sample will be taken 1 hour later. If your glucose level is high—for example, over 140 mg/dL (7.8 mmol/L)—you will be asked to take the second test, the 100-gram OGTT.

Even though your gestational diabetes will probably go away after your baby is born, you are at risk for gestational
diabetes if you become pregnant again and for type 2 diabetes later in life. You may also have a follow-up glucose tolerance test 6 to 12 weeks after your baby is born or after you stop
breast-feeding your baby. If the results of this test are normal, you will still need to be screened for diabetes at least every 3 years.

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